Eight university students are exploring caves. Deep in a wet part of the cave, they find an artefact with intricate patterns on it. One of the students, Laura Ainsell, touches the artefact. She can't remove her hand, and suddenly a shape begins to appear above the artefact.
In his laboratory, Dr Leech and his assistant Ly examine the artefact. Leech is exhausted so Ly brings him some water. Leech thinks his exhaustion comes from the same source that caused the students to be quartered in the medical wing: the artefact.
The Doctor and Mel burst in on Leech, and the Doctor offers his help. At first guards try to take him away but Leech welcomes his help. The Doctor's device causes the artefact to act strangely. Leech tells him the artefact was probably in the cave for thousands of years. The Doctor says it is alien, and asks if they've translated it. Mel and Ly look one of the drawings, and Mel recognises a jellyfish. A background whining sound has been steadily growing louder, and suddenly they all hear plopping noises outside. It is raining jellyfish.
The Doctor runs outside to investigate and sees a large spaceship, from which the jellyfish are falling. He runs back inside. Above the artefact a huge jellyfish is floating. It is a hologram. He introduces himself as Commander Hydra Sowerbii and claims the Earth for his people, the Cnidarians.
The Doctor points out that Commander Hydra's jellyfish are landing on dry land. The hologram disappears for a moment while he stops the jellyfish. While he is gone, the Doctor tries to work out a plan. One of the guards, Baines, reports that jellyfish are in the lake. Commander Hydra returns, and the Doctor suggests they talk. The Commander complies by sending a metal tentacle into the room to grab the Doctor. He is brought up to the ship.
Baines and the other guards leave to deal with the jellyfish outside. Leech collapses, and Mel and Ly take him to the medical wing.
In the Cnidarian ship, the Doctor is given an air bubble. In the medical wing, Mel sees that seven of the university students are in beds. They look awful; suddenly Laura sits up and demands to be put in water. Mel and Ly see that she and the others are developing tentacles. Mel wonders why there are only seven students present; eight were part of the exploration party. Ly answers that she was the eighth.
On the ship, the Doctor tells the Commander that he cannot have Earth. When the Commander throws him across the room, the Doctor accidentally bites his tongue. He realises something important...
Ly explains to Mel that the eight students had their life forces absorbed by Commander Hydra. She also has been giving Leech zooplankton in his water, changing him into one of them too. She advances on Mel, who backs away and grabs something behind her. She throws it at Ly, who screams. It was a bag of saline.
Mel contacts Baines and tells him the jellyfish are vulnerable to salt. On the ship, the Doctor informs Hydra that Earth's seas are salt water, but the Cnidarians are fresh water. Mel then contacts the ship and tells Hydra that Ly wants to come aboard. Ly is brought aboard, where she dissolves a pack of table salt in the ship's water.
The artefact is returned to the ship, and the Cnidarians leave Earth. The students are recovering, thanks to saline drips. The Doctor and Mel leave, with the Doctor making bad jokes.
- Commander Hydra's ship is the Water Carrier.
- Hydra plans to rename Earth "New Aqua".
- After defeating Hydra, the Doctor suggests that he and Mel get some jelly and ice cream.
- Mel was once stung by a jellyfish at Bournemouth.
- The stories in Short Trips: Zodiac are inspired by the signs of the zodiac. This story's theme is Aquarius.
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